Certain subscribers have more than one telephone line, and telephone sets connected to them which are within ringing hearing range of each other. For example, some homes have a main telephone line and a second telephone line for teenage children. When the telephones associated with those lines ring, it is usually difficult to tell which of the telephone lines is ringing. Sometimes different kinds of telephones are purchased for connection to the different lines, the different telephone sets exhibiting different sounds. For example, the telephone sets connected to one telephone line may have a bell, while those connected to the second telephone line may have electronic ringing. In this way the subscribers can tell which of the lines is ringing, and the appropriate person answer the call. This of course imposes severe restrictions on the mixing and matching of telephones and telephone sets having special features.
In addition, as described in Canadian patent application serial number 2,081,125 filed Oct. 22, 1992, invented by Deborah L. Pinard and Graham Wilson, a subscriber may invoke a call transfer to another line which may be on the same or on a different switching system. Calls to that subscriber are automatically rung on a different line. However there is no way to distinguish whether a call rung on the target line is intended for the subscriber registered against that line, or for the call designated for the transfer subscriber.
Some current switching systems allow a single subscriber to have different directory numbers which terminate on the same line, a call to either resulting in a different ringing cadence. In addition, party lines allow different ringing to occur on the same line. The same telephone set or sets connected to the single line are all rung with the different ringing cadences. However neither of these solve the problems identified above, since the different directory numbers are registered to a single subscriber, and ring only a single line which can be used at a time, or are registered to different subscribers, but again ring only a single line.